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At-The-Market Offering (Details) (USD $)
Share data in Millions, unless otherwise specified
 1 Months Ended 3 Months Ended
Apr. 30, 2015
May 31, 2014
Mar. 31, 2015
Apr. 20, 2015
At-The-Market Offering        
Aggregate sales proceeds from issue and sell of shares of common stock under sales agreement with Stifel $ 75,000,000us-gaap_ProceedsFromIssuanceOfCommonStock   $ 33,665,000us-gaap_ProceedsFromIssuanceOfCommonStock $ 21,500,000us-gaap_ProceedsFromIssuanceOfCommonStock
Commission percentage paid   2.00%mnta_CommissionPercentageOfGrossProceedsFromSaleOfCommonStock    
Aggregate stock sold pursuant to the sales agreement (in shares) 1.4mnta_StockSoldDuringPeriodSharesNewIssues   4.1mnta_StockSoldDuringPeriodSharesNewIssues  
Aggregate receivable from sale of common stock under sales agreement with Stifel     52,000,000mnta_ReceivableFromIssuanceOfCommonStock  
Expected | Maximum        
At-The-Market Offering        
Aggregate sales proceeds from issue and sell of shares of common stock under sales agreement with Stifel   75,000,000us-gaap_ProceedsFromIssuanceOfCommonStock
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